feat(07-02): implement sendMessage async thunk with optimistic updates

Add sendMessage async thunk with complete optimistic UI flow:
- pending: Adds optimistic message immediately with temp ID
- fulfilled: Replaces optimistic message with real server response
- rejected: Removes optimistic message and sets error
- Optimistic messages marked with isOptimistic flag
- Channel initialization if not exists
- Preserves existing messages during replace/remove operations

All 53 unit tests passing.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/store/features/sessionChatSlice.js

@@ -1,5 +1,5 @@
 import { createSlice, createAsyncThunk } from '@reduxjs/toolkit';
-import { getChatMessages } from '../../helpers/rest';
+import { getChatMessages, sendChatMessage } from '../../helpers/rest';
 
 /**
  * Async thunk to fetch chat history for a channel
@@ -20,6 +20,28 @@ export const fetchChatHistory = createAsyncThunk(
   }
 );
 
+/**
+ * Async thunk to send a chat message with optimistic UI updates
+ * @param {Object} params - Request parameters
+ * @param {string} params.channel - Channel ID (session ID, lesson ID, or 'global')
+ * @param {string} params.sessionId - Session ID for session channels
+ * @param {string} params.message - Message content
+ * @param {string} params.optimisticId - Temporary ID for optimistic update
+ * @param {string} params.userId - Current user ID
+ * @param {string} params.userName - Current user name
+ */
+export const sendMessage = createAsyncThunk(
+  'sessionChat/sendMessage',
+  async ({ channel, sessionId, message }) => {
+    const response = await sendChatMessage({
+      channel: sessionId ? 'session' : 'global',
+      sessionId,
+      message
+    });
+    return response;
+  }
+);
+
 /**
  * Initial state for session chat
  * @type {Object}
@@ -198,6 +220,66 @@ const sessionChatSlice = createSlice({
         const channel = action.meta.arg.channel;
         state.fetchStatus[channel] = 'failed';
         state.fetchError[channel] = action.error.message;
+      })
+      // sendMessage pending - optimistic update
+      .addCase(sendMessage.pending, (state, action) => {
+        state.sendStatus = 'loading';
+        state.sendError = null;
+
+        // Optimistic update: add message immediately
+        const { channel, message, optimisticId, userId, userName } = action.meta.arg;
+        if (!state.messagesByChannel[channel]) {
+          state.messagesByChannel[channel] = [];
+        }
+
+        state.messagesByChannel[channel].push({
+          id: optimisticId,
+          senderId: userId,
+          senderName: userName,
+          message,
+          createdAt: new Date().toISOString(),
+          channel,
+          isOptimistic: true
+        });
+      })
+      // sendMessage fulfilled - replace optimistic message
+      .addCase(sendMessage.fulfilled, (state, action) => {
+        state.sendStatus = 'succeeded';
+        state.sendError = null;
+
+        // Replace optimistic message with real one
+        const { channel, optimisticId } = action.meta.arg;
+        const realMessage = action.payload.message;
+
+        const messages = state.messagesByChannel[channel];
+        if (messages) {
+          const index = messages.findIndex(m => m.id === optimisticId);
+          if (index !== -1) {
+            messages[index] = {
+              id: realMessage.id,
+              senderId: realMessage.sender_id,
+              senderName: realMessage.sender_name,
+              message: realMessage.message,
+              createdAt: realMessage.created_at,
+              channel: realMessage.channel
+            };
+          }
+        }
+      })
+      // sendMessage rejected - remove optimistic message
+      .addCase(sendMessage.rejected, (state, action) => {
+        state.sendStatus = 'failed';
+        state.sendError = action.error.message;
+
+        // Remove optimistic message on failure
+        const { channel, optimisticId } = action.meta.arg;
+        const messages = state.messagesByChannel[channel];
+        if (messages) {
+          const index = messages.findIndex(m => m.id === optimisticId);
+          if (index !== -1) {
+            messages.splice(index, 1);
+          }
+        }
       });
   }
 });